In Story mode on Realistic difficulty, there is a glitch on the level Ambush. Make your way to the first bomb you come up to, tell your team to disarm it.
While they're doing that, plant a C4 on the right window in front of the bomb. Then turn left, look up top on the turret, detonate the C4, and you will see the enemies respawn.
You can keep killing them, and they will just keep respawning, allowing you to get an enormous amount of experience, and A.C.E.S.
Do this as long as you wish, you should get an LMG for more ammo, and all you have to do when you run out is restart the checkpoint. Enjoy.